Conventionally, in an image forming apparatus of, for example, the type which has a photosensitive drum and forms an image by an electrophotographic process, it is necessary to appropriately (typically, uniformly) charge a photosensitive drum in any. environmental conditions, in order to always secure stable image quality. To this end, charged potential of the photosensitive drum is measured by using an electric potential measuring device (electric potential sensor), and a feedback control is carried out by utilizing a result of the measurement so as to secure uniform electric potential distribution over the photosensitive drum.
For the conventional electric potential sensor, a non-contact electric potential sensor is known, and especially, the electric potential sensor of the so-called mechanically modulated alternating electric field induction type is frequently used. In this type of potential sensor, electric potential on a surface of a measuring object is a function of the amplitude of current derived from a detection electrode contained in the potential sensor, and the current is mathematically expressed by the following equation:i=dQ/dt=d[CV]/dt  (1)where Q is an amount of charge appearing on the detection electrode, C is a coupling capacitance between the detection electrode and the measuring object, and V is an electric potential on a surface of the measuring object. The coupling capacitance C is given by the following equation:C=AS/x   (2)where A is a proportional constant, which includes the dielectric constant of material, S is an area of the detection electrode, and x is a distance between the detection electrode and the measuring object.
An electric potential V on a surface of the measuring object is measured by using the relation among those items. It is known that to exactly measure a charge amount Q appearing on the detection electrode, it is preferable to periodically modulate the magnitude of the capacitance C between the detection electrode and the measuring object. The following methods for modulating the magnitude of the capacitance C are known.
A first modulating method is to effectively modulate the area S of the detection electrode. As for a typical example of this method, a fork-shaped shutter is placed between the measuring object and the detection electrode. The degree of shutting-off of lines of electric force which reaches the detection electrode from the measuring object is varied by periodically moving the shutter in a direction parallel to a surface of the measuring object. In this way, the area of the detection electrode is effectively varied to realize the modulation of the capacitance C between the measuring object and the detection electrode (see U.S. Pat. No. 4,720,682).
In another example of the modulating method, a shielding member with an aperture is disposed at a place facing the measuring object. A detection electrode is provided at a tip of a vibrating element shaped like a fork. The detection electrode is displaced just under the aperture in a direction parallel to the measuring object. In this way, the number of lines of electric force reaching the detection electrode is modulated to thereby modulate the capacitance C (see U.S. Pat. No. 3,852,667).
A second modulating method is to periodically vary a distance x between the detection electrode and the measuring object. In a typical example of this method, a detection electrode is provided at a tip of a cantilever-like vibrating element. A distance X between the measuring object and the detection electrode is periodically varied by vibrating the cantilever-like vibrating element, whereby the capacitance C is modulated (see U.S. Pat. No. 4,763,078).
Further, U.S. Pat. No. 5,212,451 discloses an electrostatic measurement apparatus having a single balanced beam.
More specifically, it discloses the following apparatus. That is, there is disclosed an apparatus for measuring the magnitude of an electrostatic field, comprising:                a balanced beam vibratory element;        means for resiliently supporting said balanced beam vibratory element;        drive means for vibrating said balanced beam vibratory element; and an electrode, operatively associated with said balanced beam vibratory element, for sensing a capacitive coupling relationship with the electrostatic field and thereby producing a signal indicative of the magnitude of the electrostatic field during modulation of the coupling relationship.        
In the technical circumstances as stated above, recently, with the trend of reduction of the photosensitive drum diameter and increase of the density of the arrangement of related components disposed around the photosensitive drum, the size reduction and the thinning of the electric potential sensor are required. In the currently used potential sensor of the mechanically modulated alternating electric field induction type, an internal volumetric space of the sensor structure is almost occupied by the component parts of the drive mechanisms for driving the fork-shaped shutter or for vibrating the cantilever-like vibrating element, and others. Accordingly, the size reduction of those drive mechanisms is essential for reducing the size of the electric potential sensor.
The current output as an output signal from the potential sensor of the mechanically modulated alternating electric field induction type is obtained from the following equation based on the equations (1) and (2):i=d[AVS/x]/dt   (3)As described above, with size reduction of the electric potential sensor, the area S of the detection electrode becomes small. As a result, the sensor output current “i” also becomes small, and the output signal from the sensor is easily influenced by external noise. The sensor is structured as an assembly of individual component parts. Thus, problem remains unsolved in terms of the size and cost reduction.